Poly(ethylene terephthalate co ethylene isophthalate) - relationship between physical properties and chemical structures

Abstract

A series of copolyesters based on different molar ratios of polyethylene terephthalate (PET) to polyethylene isophthalate (PEI) have been synthesized. With the involvement of PEI, the copolyesters become less crystallizable and even amorphous when the composition of PEI is above 20%. The WAXD profiles of the crystallizable copolyesters infer that the crystals come from PET homopolymer. DSC cooling runs indicated that the copolyesters with PEI no more than 15% are easily crystallizable; while the copolyester with 20% PEI is not easily crystallized at a cooling rate above 5 degrees C/min. Heating runs revealed that the copolyesters with PEI below 20% show a melting process. Non-uniform results were provided by WAXD and DSC; however, the effect of PEI on crystallization was deduced similarly from WAXD and DSC. Glass transition temperatures were measured by DSC. Due to flexibility of the PEI chain, glass transition temperatures of the copolyesters decrease with increasing composition of PEI, as predicted by the Fox equation and additive principle, however, the former is better in describing the relationship between T-g and composition. (C) 1999 Elsevier Science Ltd. All rights reserved.